The 4 speed and the 5 speed gearboxes are not necessarily the same ratios. The standar ratio use for first in the 4 speeds was 2.97 where most of the 5 speed T9s run 3.65 in first. You can swap the gear sets to match the long first gear of the 4 speed (my preference). I don't really use 5th under 70mph. For how you describe your driving I don't know why you'd bother.
I'd agree with having the higher ratio 1st gear, it definitely improved my box, but I wouldn't say 5th is useless under 70mph, it makes for a very relaxed bimbling gear - always assuming the engine is making enough power to pull it, of course.
